Revert problem.relativistic-limo.tex back to expecting `limo.*`.

Revert problem.relativistic-limo.tex back to expecting `limo.*`.

This reverses a portion of

  commit ed3656403a9befab65e394949f3862657659f41d
  Author: W. Trevor King <wking@drexel.edu>
  Date:   Wed Apr 11 21:04:46 2012 -0400

    Make the Python limo script executable.

    Also adjust the name of the expected file to ${SCRIPT/.py/} to
    make it easier to automatically generate the required image.

The new scheme works with

  TARGET="$(echo "${SCRIPT}" | awk -F. '{print $(NF-1)}').png"
  "${SCRIPT}" "${TARGET}"

This avoids problems like

  LaTeX Warning: File `problem.relativistic-limo.limo' not found on
  input line 50 .
  ! LaTeX Error: Unknown graphics extension: .relativistic-limo.limo.

With the new rule, if you write a problem `abcd.efgh.tex` that needs
an image `xyz.png`, the script generating that image should be
`abde.efgh.xyz.py`.